It is known in the pump art to provide magnetic drive pumps incorporating a housing completely enclosing the impeller and related structure. That is, no rotating shaft portions extend completely through the housing, eliminating the need for seals or packings of the type required in conventional pumps wherein the shaft extends through the housing at at least one end thereof. Consequently, all adjustments in the positioning of an impeller within the housing must be made prior to final assembly of the pump.
In the case of magnetically driven, sealess pumps of the type just described, a shoulder on the center of the impeller bears against the housing. The end of the impeller support shaft remote from the impeller is conventionally simply fixed at a preselected position within the pump housing, for example at the housing structure portion accommodating the driven magnetic structure of the pump. The clearance between the impeller and the volute is therefore fixed in such prior art arrangements. Pump efficiency is very sensitive to clearance between the impeller and the volute of the pump housing and prior art magnetically driven, sealess pumps constructed as just described have widely varying efficiencies when they come out of the factory door. Of course, the user of the pump has no way to conveniently adjust impeller placement because of the sealed nature of the pump device.